The solar wind, a steady stream of charged subatomic particles that stream out from the sun at a speed of one million miles per hour, has dwindled to its weakest state since recording began, researchers say. While researchers already knew that solar winds fluctuate in 11-year cycles, the current doldrums trump the declines seen over the past 50 years. “We know that the sun has been this cool before, this inactive before,” said [physicist] Nancy Crooker…. “But that was prior to the Space Age, so we didn’t have actual physical measurements until now” [SPACE.com].

The data was collected by the first solar explorer, the Ulysses probe, which was launched in 1990 as a joint venture between NASA and the European Space Agency. The 17-year-old space probe, which circles the sun from a distance of about 337 million miles, has been studying the environment above and below the poles of the sun. It is just months away from shutting down because of freezing fuel [AP].

The findings, published in Geophysical Research Letters [subscription required], give researchers new insights into our local star‘s behavior, and how it affects conditions throughout the solar system. The sun’s current state suggests that the heliosphere — the vast, protective magnetic cavity carved by the solar wind — has temporarily shriveled. This decline lets more harmful galactic cosmic rays into the solar system and makes it riskier for astronauts traveling beyond Earth’s own, much smaller magnetic bubble, or magnetosphere [Science News]. However, the International Space Station is well within the Earth’s protective magnetosphere, and there are no immediate missions to send astronauts to the moon or beyond.

The temporary shrinking of the heliosphere appears to have been confirmed by the two Voyager spacecraft that are currently traveling towards the outer edge of the solar system, and have already passed through a region called the termination shock where the solar wind smashes into interstellar space. Voyager 2, which was trailing behind Voyager 1, encountered the termination shock at a closer distance to the sun than its predecessor, suggesting the heliosphere has indeed shrunk in the intervening years [Science News].